Alarm circuit

ABSTRACT

A solid state circuit for indicating that an alarm condition exists at a remote location or that an interruption in the electrical connections to the remote location has occurred. The circuit includes a normally-open contact at the remote location connected in circuit with a resistor bridge circuit at a control location, closure of the contact causing a voltage unbalance in the bridge circuit to cause a programmable unijunction transistor to switch to a conducting state and operate an alarm lamp.

United States Patent '[1 1 Ranganath 1 ALARM CIRCUIT [75] Inventor: Gorur Ramaswmiengar Ranganath, Springfield, Mass.

{73] Assignee: Standard Electric Time Corporation, Springfield, Mass.

[22] Filed: Nov. 19, 1971 [21] Appl. No.: 200,365

[52] US. Cl. 340/409, 340/276 [51] Int. Cl. G08b 23/00 [58] Field of Search 340/409, 276

[56] References Cited UNITED STATES PATENTS 3,252,156 5/1966 Muether 340/276 3,500,394 3/1970 Egesdal 340/409 [451 July 17, 1973 3,641,552 2/1972 Friberg 340/409 X 3,430,231 2/1969 Weld 340/409 3,478,352 11/1969 Eisenberg 340/409 Primary Examiner-David L. Trafton A ttorney Arthur J Wagner et a1.

[ ABSTRACT A solid state circuit for indicating that an alarm condition exists at a remote location or that an interruption in the electrical connections to the remote location has occurred. The circuit includes a normally-open contact at the remote location connected in circuit with a resistor bridge circuit at a control location, closure of the contact causing a voltage unbalance in the bridge circuit to cause a programmable unijunction transistor to switch to a conducting state and operate an alarm lamp.

5 Claims, 3 Drawing Figures PATENIEL JUL 17 I373 sum 2 or 3 ALARM CIRCUIT This invention relates to an alarm circuit which is particularly suitable for use in a fire alarm system. Such systems are sometimes referred to as annunciator systems and, for convenience, that term will be used herein. The invention is particularly applicable to an improved annunciator system which provides indications in response :to either an alarm condition at a monitored station or a trouble condition of the annunicator system itself.

Previously-proposed annunicator systems have used electro-mechanical relays or zener diodes exclusively but such systems have been found to have considerable disadvantages in manufacture and use.

It is an object of the present invention to provide an alarm circuit which is not subject to all the disadvantages of the above-mentioned annunicator systems using electro-mechanical relays.

Accordingly, the present invention provides an alarm circuit comprising at least one normally-open contact connected in parallel with a first resistor in one branch of a resistor bridge circuit having at least two branches,-

an alarm condition causing at least one normally-open contact to close, a first programmable unijunction transistor having a first anode electrode, a first anode gate electrode and a first cathode electrode, a first of said first electrodes being connected to a point in said one branch, a second of said first electrodes being connected to a point in the other of said two branches, and the third of said first electrodes being connected to a point in an alarm indicating circuit, whereby said programmable unijunction transistor is switched from a first state to a second state when said one normallyopen contact is closed to cause said alarm indicating circuit to provide an alarm indication.

One previously proposed annunicator system is described and illustrated in G. E. Fosters U .8. Pat. No. 3,124,793 (issued Mar. 10, 1964) and this system generally comprises a plurality of pairs of normally open contacts, each pair of contacts being associated with one monitored device at a remotely located station; an equal plurality of supervising circuits, each including an alarm indicating lamp and circuitry for energizing the lamp when an associated monitored device exhibits an alarm condition; and a common alert device and circuitry for energizing the alert device when any monitored device exhibits an alarm condition. The system described in said United States Patent does not however provide indications so that the operator may distinguish between an alrm condition at the monitored device and a trouble condition, such as an open circuit, of the system itself.

It is an object of an embodiment of the present invention to provide an annunicator system capable of providing indications to an operator at a central location as to both an alarm condition of a monitored device or environment and a trouble condition of the annunciator system itself, thereby overcoming the noted disadvantage of the annunciator system described in said United States patent.

Accordingly, a specific embodiment of the present invention comprises at least one normally-open contact connected in parallel with a resistor in one branch of a resistor bridge circuit having two branches, an alarm condition causing at least one normally-open contact to close, a first programmable unijunction transistor having an anode electrode connected to a point in said one branch and an anode gate electrode connected to a point in the other of said two branches and a cathode electrode connected to a point in a first alarm indicating circuit, a second programmable unijunction transistor having an anode gate electrode connected to a point in said one branch and an anode electrode connected to a point in said other branch and a cathode electrode connected to a point in a second alarm indicating circuit, whereby said first programmable unijunction transistor is normally non-conducting and when said one normally-open contact is closed said first programmable unijunction transistor is switched to a conducting state to cause said first alarm indicating circuit to provide an alarm indication, and said second programmable unijunction transistor is normally nonconducting and when a connection between any normally-open contact and the remainder of said one branch becomes open circuited said second programmable unijunction transistor is switched to a conducting state to cause said second alarm indicating circuit to provide an alarm indication.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of an annunciator system according to a specific embodiment of the present invention;

FIG. 2 is a schematic circuit diagram of a preferred embodiment of the invention; and,

FIG. 3 is a schematic circuit diagram of an alternative embodiment of the present invention.

Referring now to FIG. 1, there is shown, by way of example, three remote station monitoring circuit units 2, 4 and 6 and a central station equipment unit 8 including typical supervisory circuits 10 and 12 connected to a common control unit 14 incorporating an alarm lamp 16 and a trouble lamp 18 forming part of an alarm indicating circuit.

In FIG. 2 there is illustrated a schematic circuit diagram of a preferred embodiment of the invention and it will be seen that each remote station monitoring circuit 2 includes a normally open contact 20 connected in parallel with a resistor 22. The connection leads 24 and 26 from the terminals 28 and 30 of the supervisory circuit 10 include a respective one of two normally closed contacts 32 and 34.

The control circuit 10 in FIG. 2 includes one branch of a resistor bridge circuit including the resistor 22 connected between terminals 28 and 30 and resistor 36 connected from termianl 30 to the negative potential line 38 of the circuit, terminal 28 being connected to the positive potential line 40 of the circuit. The other branch of the resistor bridge circuit includes resistors 42, 44 and 46 connected in series between the negative potential line 38 and the positive potential line 40.

Terminal 30 is identified as terminal A in FIG. 2 and the operation of the control unit is determined by the relationship between the voltage at terminal A in the first branch of the two branch resistor bridge circuit and the voltage at the terminals identified as B and C in the second branch of the resistor bridge circuit of control unit 10 in FIG. 2.

The anode electrode 43 of a first programmable unijunction transistor 50 is connected through a resistor 52 to terminal A(30) whilst the trigger gate electrode 54 is connected through a resistor 56 to the terminal B in the second branch of the bridge circuit. The cathode electrode 58 of the unijunction transistor 50 is connected to the junction 60 of a resistor 62 and the trigger gate electrode 64 of a silicon control rectifier 66. The anode electrode 68 of the SCR 66 is connected to the positive line 40 whilst the cathode electrode 70 of SCR 66 is connected via terminal 72 to one side of the alarm lamp 16 whose other side is connected to the negative line 38. As shown in FIGS. 1 and 2, the lamp 16 is in the control unit 14.

A second programmable unijunction transistor 80 is so connected as to be responsive to a trouble indication in the remote station monitoring circuit 2. The trigger gate electrode 82 of the programmable unijunction transistor 80 is connected through a resistor 84 to terminal 30, Le, point A, whilst the anode electrode 86 is connected through a resistor 88 to terminal 90 in the second branch of the bridge circuit, i.e. point C in FIG. 2, point B in the second branch of the bridge circuit being also identified as terminal 92.

The cathode electrode 94 of the second unijunction transistor 80 is connected to point 96 on one side of a resistor 98, having its other side connected to the negative potential line 38. Point 96 is in a connection line between the resistor 98 and the trigger gate electrode of a second silicon control rectifier 100. The anode electrode 102 of the SCR 100 is connected to the positive potential line 40 whilst the cathode electrode 104 is connected, via a terminal 105, to one side of the trouble lamp 18 (FIGS. 1 and 2), the other side of lamp 18 being connected to the negative potential line 38 in the trouble indicating circuit.

It will be apparent that a plurality of module units, i.e., supervisory circuits 10, may be associated with a particular alarm lamp and trouble lamp. For this type of operation, it is desirable to provide additional circuitry to prevent interaction between the different modules and in FIG. 2 there are illustrated diodes 106 and 108 and associated inductive components 110 and 112 for this purpose.

Referring to FIG. 2 it will be seen that a capacitor 114 is connected from point 116, between resistor 52 and programmable unijunction transistor 50, and the negative potential line 38.

The operation of the embodiment of this invention illustrated in FIG. 2 will not be considered in a situation where the illustrated embodiment of the invention is functioning as a fire alarm supervisory system. The monitoring circuit 2 is located in the area to be protected and if a fire occurs then the normally-open contact closes thereby shorting out the resistor 22. Resistors 22 and 36 are of apporximately equal size and thus the shorting of resistor 22, due to a fire alarm condition, results in an appreciable change in the voltage at terminal 30, point A, which now assumes the voltage of the positive potential line 40 without there being any voltage drop across resistor 22. Previously point A was at a lower potential than point B in the second branch of the bridge circuit, but with the shorting of contact 20, the voltage at point A rises above the voltage at point B and thus the programmable unijunction transistor 50 is caused to conduct. This causes the capacitor 114 to be discharged through the programmable unijunction transistor 50 and the resistor 62 in the gate circuit of the alarm SCR 66. There is thus a change in potential at the trigger gate electrode 64 of SCR 66 which is thus switched to a conducting state whereby current flows through the alarm lamp 16 causing it to become illuminated to provide the required alarm indication.

If trouble should develop in the monitoring circuit 2 whereby one of the normally closed contacts 32 and 34 opens or another open-circuit occurs somewhere in the unit 2, then the voltage at terminal A, point 30, immediately falls to the potential of the negative potential line 38 (since there is now no connection through to the positive potential line 40). Thus the potential on the trigger gate electrode 82 of the programmable unijunction transistor falls below the potential at point C and on the anode electrode 86 of the programmable unijunction transistor 80. Programmable unijunction transistor 80 is thus caused to conduct whereby current flows, via terminal 96, through resistor 98 in the trigger gate electrode circuit of the SCR 100. Thus SCR is switched to a conducting state whereby current flows through the trouble lamp 18 to provide the required indication that trouble exists in the monitoring circuit 2.

It will be appreciated that the selection of value of the components shown in FIG. 2 will be readily apparent. However, by way of example, typical component values of some of the illustrated components are given below.

Resistors 62 and 98 4.7Kohms, half watt Resistors 22, 36, 42, 44, 46, 52, 56, 84, and 88 47Kohms half watt, 5 percent Capacitors 114 and 0.1M, 50 v.D.C.

P.U.T. unijunction transistors 50 and 80 General Electric type D13 T SCRs 66 and 100 General Electric type C106 Al Diodes 106 and 108 General Electric type H21 A Positive line 40 24 volts DC i 20 percent partially filtered.

It will be apprecited that the present invention is not restricted to the circuit illustrated in FIG. 2 but other arrangements may readily be designed by a man skilled in the art. For example, in FIG. 3 an alternative arrangement is illustrated.

Referring to FIG. 3, it will be seen that the same reference numerals have been used as were used in FIG. 2 wherever this is practical. The circuit will again be described for use in a fire alarm system and includes a plurality of normally-open alarm initiating contacts 20 connected in parallel with a resistor 22. However, it will be seen tht the resistor 22 is not connected directly to the positive line 40 but is connected thereto through a further resistor in the control unit. Electrical connections contact between the remote circuit having the normally-open contacts 20, and the control unit 10, including the unijunction transistors 50 and 80, are again made through the terminals 28 and 30. If the control unit 10 is constructed as a printed circuit board then 8 such terminals might well be necessary to make contact with circuitry external to the board. It will be appreciated that the circuit of FIG. 3 is designed to provide an alarm indication when any normally-open alarminitiating contact 20 is closed and a trouble indication when any part of the connection lines to the contacts 20 and resistor 22 from the terminals 32 and 34 becomes open-circuit. The alarm lamp 16, the trouble lamp 18, and the associated circuitry, is illustrated in FIG. 3 and will be seen to be similar to the circuitry of FIG. 3.

In FIG. 3 the one arm of the bridge circuit includes resistor 130, resistor 22 and resistor 36, with the intermediate junctions 28 (point D) and 30 (point A) and the other arm of the bridge circuit includes resistors 42,

44, and 98 of the intermediate junctions 92 (point B) and 90 (point C).

Connection of theprogrammable unijunction transistors 50 and 80 to the first and second resistor branch of the bridge circuit is slightly different in FIG. 3, but the basic principal of operation is the same as in FIG. 2.

Referring to FIG. 3 it will be seen that the anode electrode 48 of the programmable unijunction transistor 50 is connected through resistor 52 to terminal 28 (point D), whilst the trigger gate electrode 54 is connected through resistor 56 to terminal 92 (point B). The cathode electrode 58 is connected to the junction 60 of the resistor 62 and the trigger gate electrode 64 of the SCR 66.

The anode electrode 86 of the programmable unijunction transistor 80 is connected to terminal 30 (point A) whilst the trigger electrode 82 is connected to the .junction 90 (point C) between resistor 44 and resistor 98. The cathode electrode 94 of PUT 80 is connected to the junction 96 of resistor 98 and the trigger electrode of the SCR 100.

In operation, the programmable unijunction transistor 50 compares the voltage between terminals 28 and 92 (points D and B) and unijunction transistor 80 compares the voltages between terminals 30 and 90 (points A and C). If an alarm condition does not exist in the circuit and there is no trouble in the circuit connections, then the voltage at terminal 90 is greater than the voltage at terminal 30 whilst the voltage at terminal 92 is greater than the voltage at terminal 28. The resistors 130, 22, 36, 42, 44, and 98 are so selected that, under these conditions, the programmable unijunction transistors 50 and 80 are not conducting. If an open circuit should develop in any of the connection lines between the terminals 28, 30 and resistor 22, then the voltage at terminal 28 (point D) rises above the voltage at terminal 92 (point B). Thus, programmable unijunction transistor 50 is switched to a conducting state and fires, i.e. triggers, the trouble semiconductor rectifier, SCR, 66. Thus, current is passed through lamp 1% and it is illuminated and the relay 112 is energized. It will be appreciated that if individual trouble indications are required from the system then the respective lamps 18 are connected between the positive line 40 and the junction 132. When group indications or alarms are required any external solid state device, relay etc. can be used between the positive line 40 and terminal 134. It will be understood that the output from the alarmtrouble module can be used in many different ways, for example, to operate individual lamps, relays or solid state devices; or, by using diodes, they could be grouped. Separate solid-state circuits could be provided to operate bells which also include supervision of wires connected to the bells.

If a number of so-called station circuits, say 100, are provided they may be divided into several groups. In such systems, there are not only individual indications, but also group indications as well as common indications. Common indications occur when there is an alarm on any one of the 100 station circuits.

The lamp l8 and relay 112, are, of course, external to the printed board and are illustrated merely to facilitate an understanding of the operation of the circuit. Capacitor l 14 is used to store sufficient charge to positively fire, i.e. switch the state off, the SCR 66 and to absorb any transient currents in the circuit. Resistors 52 and 56 are provided to reduce the risk of false alarms in the circuit, resistor 52 being 47Khoms in value and resistor 56 being 100 Khoms in value.

When an alarm condition exists, and one of the normally open alarm initiating contacts 20 closes, then an increase in voltage occurs at terminal 30 (point A) in relation to the voltage at terminal (point C). This causes the programmable unijunction transistor 80 to switch to a conducting state whereupon the SCR is fired, i.e. switched, to produce an appropriate alarm indication due to the illumination of the respective lamp 16.

It will be appreciated that the indicated values of the components are by way of example and render the circuit relatively insensitive to fluctuation in the supply voltage because the circuit is not dependent on the absolute of the supply voltage. The operation of the circuit is principally dependent on the relative voltages between terminals 28 and 92 as well as between the relative voltages between terminals 30 and 90, it being clear that this relationship does not vary with variation of supply voltage. Furthermore, the circuit is extremely economical because of the low relative cost of the programmable unijunction transistors etc. Some components may, in some uses, be omitted or changed in value with appropriate design modifications.

TI-Ie circuit of FIG. 2 is particularly suited for use in a grounded alarm system where the negative line 38 can be grounded. In such a grounded alarm system, if an accidental ground should occur on the extended line from terminal 30 (point A) to resistor 22 then the voltage at point A drops to substantially zero potential. Thus unijunction transistor 80 is switched to a conducting state and the trouble indicating lamp 18 is operated. On the other hand an accidental ground on the extended line from terminal 28 to resistor 22 will cause a power supply fuse to blow, and this can be convenviently caused to operate the trouble-indicating lamp 18.

The circuit of FIG. 3 can, of course, be used where the power supply is not grounded. The extended lines are balanced in this arrangement and if a ground should occur on one of them, no trouble indication is obtained.

It will be noted that the SCRs 66 and 100 are connected to the minus side of the loads (such as relays, lamps etc.) in FIG. 3 whereas they are connected to the positive side of the loads in FIG. 2. These arrangements could be used interchangeably without affecting the operation of the system. The preferred configuration with regard to SCRs is arrived at from consideration of noise in the circuit.

It will be apparent from the above description that the trouble or alarm indication is given as a result of the appropriate SCR being rendered conducting. Whether the respective SCR continues to conduct (lock in) even after the original alarm or trouble condition has been cleared, will depend on the characteristics of the DC power supply between the positive line 40 and the negative line 38. If the DC supply is partially or completely filtered, the SCR will remain conducting. To reset the circuit, it is desirable to provide a switch in the voltage supply connection whereby the supply is disconnected to de-energize that part of the circuit and de-energize any common alarm and common trouble indicating means whilst the supervisory trouble or supervisory alarm indicating means, eg. contact 20, of any of the respective supervisory circuits remain energized.

The illustrated embodiments of the invention have been described with particular reference to a fire alarm system. It will be appreciated that an annunciator system according to the present invention is not restricted thereto and other systems and circuits may readily be designed according to the present invention. Similarly, whilst lamps have been illustrated for alarm or trouble indications, the invention is not limited thereto. For example, a bell or buzzer indication may be provided as well as, or instead of a lamp.

We claim:

1. An alarm indicating circuit comprising alarm condition indicating means and first control means operable when enabled to energize said alarm condition indicating means, trouble indicating means and second control means operable when enabled to energize said trouble indicating means, at least one normally-open contact connected in parallel with a first resistor in one branch of a resistor bridge circuit having at least two branches, an alarm condition causing at least one normally-open contact to close, a first normally nonconducting programmable unijunction transistor having a first anode electrode, a first anode gate electrode and a first cathode electrode, a first one of said first electrodes being connected to a point in said one branch, a second one of said first electrodes being connected to a point in the other of said two branches, and the third of said electrodes being connected to said first control means, a second normally non-conducting programmable unijunction transistor having a second anode electrode, a second anode gate electrode and a second cathode electrode, a first of said second electrodes being connected to a point in said one branch, a second of said second electrodes being connected to a point in the other of said two branches, and the third of said second electrodes being connected to said second control means, said first programmable unijunction transistor being rendered conductive when said one normally-open contact is closed to enable said first control means for effecting energization of said alarm condition indicatlng means to indicate an alarm condition, and said second programmable unijunction transistor when an open-circuit condition occurs in the connections between said first resistor and the remainder of said bridge circuit to enable said second control means for effecting energization of said trouble indicating means to indicate an open circuit condition.

2. An alarm circuit according to claim 1 wherein said one branch of said resistor bridge circuit comprises said first resistor connected in series with a second resistor and connected at one end to the positive line of a supply voltage for the bridge circuit, and the remote end of said second resistor is connected to the negative line of the supply voltage, said other branch of the bridge circuit including third, fourth and fifth resistors connected in series between the positive and negative lines of the supply voltage, the remote end of said third resistor being connected to said positive line the anode electrode of the first programmable unijunction transistor being electrically connected to the junction of said first and second resistors, the anode gate electrode of the first programmable unijunction transistor being electrically connected to the junction of said third and fourth resistors and the cathode electrode of said first programmable unijunction transistor being electrically connected to said first control means, the anode gate electrode of the second programmable unijunction transistor being electrically connected to the junction of said first and second resistors, the anode electrode of said second programmable unijunction transistor being electrically connected to the junction of said third and fourth resistors and the cathode electrode of said second programmable unijunction transistor being electrically connected to said second control means.

3. An alarm circuit according to claim 1 wherein said one branch of said resistor bridge circuit comprises said first resistor connected in series with a second resistor and connected at one end through a third resistor to the positive line of a supply voltage for the bridge circuit, and the remote end of said second resistor is connected to the negative line of the supply voltage, said other branch of the bridge circuit including fourth, fifth and sixth resistors connected in series between the positive and negative lines of the supply voltage, the remote end of said fourth resistor being connected to said positive line, the anode electrode of the first programmable unijunction transistor being electrically connected to the junction of said first and third resistors, the anode gate electrode of the first programmable unijunction transistor being electrically connected to the junction of said fourth and fifth resistors and the cathode electrode of said first programmable unijunction transistor being electrically connected to said first control means, the anode gate electrode of the second programmable unijunction transistor being electrically connected to the junction of said fifth and sixth resistors, the anode electrode of said second programmable unijunction transistor being electrically connected to the junction of said first and second resistors and the cathode electrode of said second programmable unijunction transistor being electrically connected to said second control means.

4. An alarm circuit according to claim 2 wherein said alarm condition indicating means includes a first lamp and said trouble indicating means includes a second lamp.

5. An alarm circuit according to claim 2 wherein said second programmable unijunction transistor is rendered conductive when an accidental ground connection occurs in said connections between said first resistor and the remainder of said bridge circuit.

yggy UNITED STATES PATENT OFFICE CERTIFICATE OF CURRECTIQN I Patent No. 3 ,747 ,093 Dated July 17 1973 inventgfls) Gorur Ramaswmiengar Ranganath It is certified that: error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: I

Column 7, line 45, after "-sistor" insert being rendered conductive Signed and sealed this 1st day of January 1974.

(SEAL) Attest:

EDWARD M. FLETCHERYJR I RENE D. TEGTMEYER,

Acting Commissioner of Patents Attesting Officer 

1. An alarm indicating circuit comprising alarm condition indicating means and first control means operable when enabled to energize said alarm condition indicating means, trouble indicating means and second control means operable when enabled to energize said trouble indicating means, at least one normallyopen contact connected in parallel with a first resistor in one branch of a resistor bridge circuit having at least two branches, an alarm condition causing at least one normally-open contact to close, a first normally non-conducting programmable unijunction transistor having a first anode electrode, a first anode gate electrode and a first cathode electrode, a first one of said first electrodes being connected to a point in said one branch, a second one of said first electrodes being connected to a point in the other of said two branches, and the third of said electrodes being connected to said first control means, a second normally non-conducting programmable unijunction transistor having a second anode electrode, a second anode gate electrode and a second cathode electrode, a first of said second electrodes being connected to a point in said one branch, a second of said second electrodes being connected to a point in the other of said two branches, and the third of said second electrodes being connected to said second control mEans, said first programmable unijunction transistor being rendered conductive when said one normally-open contact is closed to enable said first control means for effecting energization of said alarm condition indicatIng means to indicate an alarm condition, and said second programmable unijunction transistor when an open-circuit condition occurs in the connections between said first resistor and the remainder of said bridge circuit to enable said second control means for effecting energization of said trouble indicating means to indicate an open circuit condition.
 2. An alarm circuit according to claim 1 wherein said one branch of said resistor bridge circuit comprises said first resistor connected in series with a second resistor and connected at one end to the positive line of a supply voltage for the bridge circuit, and the remote end of said second resistor is connected to the negative line of the supply voltage, said other branch of the bridge circuit including third, fourth and fifth resistors connected in series between the positive and negative lines of the supply voltage, the remote end of said third resistor being connected to said positive line the anode electrode of the first programmable unijunction transistor being electrically connected to the junction of said first and second resistors, the anode gate electrode of the first programmable unijunction transistor being electrically connected to the junction of said third and fourth resistors and the cathode electrode of said first programmable unijunction transistor being electrically connected to said first control means, the anode gate electrode of the second programmable unijunction transistor being electrically connected to the junction of said first and second resistors, the anode electrode of said second programmable unijunction transistor being electrically connected to the junction of said third and fourth resistors and the cathode electrode of said second programmable unijunction transistor being electrically connected to said second control means.
 3. An alarm circuit according to claim 1 wherein said one branch of said resistor bridge circuit comprises said first resistor connected in series with a second resistor and connected at one end through a third resistor to the positive line of a supply voltage for the bridge circuit, and the remote end of said second resistor is connected to the negative line of the supply voltage, said other branch of the bridge circuit including fourth, fifth and sixth resistors connected in series between the positive and negative lines of the supply voltage, the remote end of said fourth resistor being connected to said positive line, the anode electrode of the first programmable unijunction transistor being electrically connected to the junction of said first and third resistors, the anode gate electrode of the first programmable unijunction transistor being electrically connected to the junction of said fourth and fifth resistors and the cathode electrode of said first programmable unijunction transistor being electrically connected to said first control means, the anode gate electrode of the second programmable unijunction transistor being electrically connected to the junction of said fifth and sixth resistors, the anode electrode of said second programmable unijunction transistor being electrically connected to the junction of said first and second resistors and the cathode electrode of said second programmable unijunction transistor being electrically connected to said second control means.
 4. An alarm circuit according to claim 2 wherein said alarm condition indicating means includes a first lamp and said trouble indicating means includes a second lamp.
 5. An alarm circuit according to claim 2 wherein said second programmable unijunction transistor is rendered conductive when an accidental ground connection occurs in said connections between said first resistor and the remainder of said bridge circuit. 